This invention relates to electrochemical fuel cells. More particularly, this invention relates to electrochemical fuel cells which employ hydrogen as a fuel and receive an oxidant to convert the hydrogen to electricity and heat, and which utilize a proton exchange membrane as the electrolyte.
Generally, a fuel cell is a device which converts the energy of a chemical reaction into electricity. It differs from a battery in that the fuel cell can generate power as long as the fuel and oxidant are supplied.
A fuel cell produces an electromotive force by bringing the fuel and oxidant into contact with two suitable electrodes and an electrolyte. A fuel, such as hydrogen gas, for example, is introduced at a first electrode where it reacts electrochemically in the presence of the electrolyte and catalyst to produce electrons and cations in the first electrode. The electrons are circulated from the first electrode to a second electrode through an electrical circuit connected between the electrodes. Cations pass through the electrolyte to the second electrode. Simultaneously, an oxidant, typically air, oxygen enriched air or oxygen, is introduced to the second electrode where the oxidant reacts electrochemically in presence of the electrolyte and catalyst, producing anions and consuming the electrons circulated through the electrical circuit; the cations are consumed at the second electrode. The anions formed at the second electrode or cathode react with the cations to form a reaction product. The first electrode or anode may alternatively be referred to as a fuel or oxidizing electrode, and the second electrode may alternatively be referred to as an oxidant or reducing electrode. The half-cell reactions at the two electrodes are, respectively, as follows:
H2xe2x86x922H+=2exe2x88x92
xc2xdO2+2H++2exe2x88x92xe2x86x92H2O
The external electrical circuit withdraws electrical current and thus receives electrical power from the cell. The overall fuel cell reaction produces electrical energy which is the sum of the separate half-cell reactions written above. Water and heat are typical by-products of the reaction.
In practice, fuel cells are not operated as single units. Rather, fuel cells are connected in series, stacked one on top of the other, or placed side by side. A series of fuel cells, referred to as fuel cell stack, is normally enclosed in a housing. The fuel and oxidant are directed through manifolds to the electrodes, while cooling is provided either by the reactants or by a cooling medium. Also within the stack are current collectors, cell-to-cell seals and, insulation, with required piping and instrumentation provided externally of the fuel cell stack. The stack, housing, and associated hardware make up the fuel cell module.
Fuel cells may be classified by the type of electrolyte, either liquid or solid. The present invention is primarily concerned with fuel cells using a solid electrolyte, such as a proton exchange membrane (PEM). The PEM has to be kept moist with water because the available membranes will not operate efficiently when dry. Consequently, the membrane requires constant humidification during the operation of the fuel cell, normally by adding water to the reactant gases, usually hydrogen and air.
The proton exchange membrane used in a solid polymer fuel cell acts as the electrolyte as well as a barrier for preventing the mixing of the reactant gases. An example of a suitable membrane is a copolymeric perfluorocarbon material containing basic units of a fluorinated carbon chain and sulphonic acid groups. There may be variations in the molecular configurations of this membrane. Excellent performances are obtained using these membranes if the fuel cells are operated under fully hydrated, essentially water-saturated conditions. As such, the membrane must be continuously humidified, but at the same time the membrane must not be over humidified or flooded as this degrades performances. Furthermore, the temperature of the fuel cell stack must be kept above freezing in order to prevent freezing of the stack.
Cooling, humidification and pressurization requirements increase the cost and complexity of the fuel cell, reducing its commercial appeal as an alternative energy supply in many applications. Accordingly, advances in fuel cell research are enabling fuel cells to operate without reactant conditioning, and under air-breathing, atmospheric conditions while maintaining usable power output.
The current state-of-the-art in fuel cells, although increasingly focusing on simplified air-breathing, atmospheric designs, has not adequately addressed operations in sub-zero temperatures, which requires further complexity of the design. For instance, heat exchangers and thermal insulation are required, as are additional control protocols for startup, shut-down, and reactant humidifiers.
Where a solid polymer proton exchange membrane (PEM) is employed, this is generally disposed between two electrodes formed of porous, electrically conductive material. The electrodes are generally impregnated or coated with a hydrophobic polymer such as polytetrafluoroethylene. A catalyst is provided at each membrane/electrode interface, to catalyze the desired electrochemical reaction, with a finely divided catalyst typically being employed. The membrane electrode assembly is mounted between two electrically conductive plates, each which has at least one flow passage formed therein. The fluid flow conductive fuel plates are typically formed of graphite. The flow passages direct the fuel and oxidant to the respective electrodes, namely the anode on the fuel side and the cathode on the oxidant side. The electrodes are electrically coupled in an electric circuit, to provide a path for conducting electrons between the electrodes. In a manner that is conventional, electrical switching equipment and the like can be provided in the electric circuit. The fuel commonly used for such fuel cells is hydrogen, or hydrogen rich reformate from other fuels (xe2x80x9creformatexe2x80x9d refers to a fuel derived by reforming a hydrocarbon fuel into a gaseous fuel comprising hydrogen and other gases). The oxidant on the cathode side can be provided from a variety of sources. For some applications, it is desirable to provide pure oxygen, in order to make a more compact fuel cell, reduce the size of flow passages, etc. However, it is common to provide air as the oxidant, as this is readily available and does not require any separate or bottled gas supply. Moreover, where space limitations are not an issue, e.g. stationary applications and the like, it is convenient to provide air at atmospheric pressure. In such cases, it is common to simply provide channels through the stack of fuel cell for flow of air as the oxidant, thereby greatly simplifying the overall structure of the fuel cell assembly. Rather than having to provide a separate circuit for oxidant, the fuel cell stack can be arranged simply to provide a vent, and possibly, some fan or the like, to enhance air flow.
There are various applications for which humidification of fuel cells poses particular problems and challenges. For example, operation of fuel cells in mobile vehicles usually means that there is no readily available supply of water for humidifying incoming oxidant and fuel streams. It is usually undesirable to have to provide water to a vehicle for this purpose and also to have to carry the excess weight of the water around in the vehicle. In contrast, for stationary applications, providing a supply of water for humidification is usually quite possible.
However, there also some stationary applications for which humidification is not straightforward. For example, fuel cells are often used to provide power supplies to remote sensing equipment, in locations where water may not be readily available. Additionally, such remote use of fuel cells often occurs at locations with extreme climatic conditions. Thus, it has been known to use fuel cell stacks in the Antarctic regions and the like, for providing supply to scientific instruments. It is simply not realistic to provide a separate supply of water for humidification, because of the problems of preventing freezing of the water supply. Additionally, ambient air used as an oxidant is excessively dry, so that humidification is more critical than when using relatively moist air at more moderate temperatures. It will be appreciated that similar extreme conditions can be found in desert locations and the like.
Accordingly, the present invention is based on the realization that, as a fuel cell inherently produces excess moisture or water as a waste product, this water is available for recycling to humidify in coming flows to the fuel cell.
More particularly, the present inventors have realized that it is advantageous to recover water from the waste or outlet flows from a fuel cell or fuel cell stack, so as to avoid having to provide a separate water source to humidify the oxidant and/or fuel streams.
It has also been recognized that, in extreme climatic conditions, it is desirable, and even in some situations essential, that the humidity of discharged fuel and/or oxidant streams be below certain levels. For example, in extremely cold conditions, if the discharged streams contain significant moisture levels, then this moisture can immediately freeze. In practice, this will form a mist or fog or fine droplets or ice pellets, which would tend to build up on the outside of the apparatus. It will be appreciated that, for a stationary installation intended to provide power supplies to scientific instruments over a long period of time, such a possibility is highly undesirable, and could lead to blockage of vents, undesirable loading due to build-up of ice and other problems. For these reasons, it is desirable that discharged streams contain reduced levels of moisture.
In accordance with a first aspect of the present invention, there is provided a fuel cell comprising: an anode with a respective anode inlet and an anode outlet for a fuel gas; a cathode with a respective cathode inlet for an incoming oxidant gas stream and a cathode outlet for an outgoing oxidant gas stream; an electrolyte between the anode and the cathode; first and second dryers; and valve means connecting the first and second dryers to the cathode inlet and the cathode outlet, whereby, in use, the first dryer can be connected to one of the cathode inlet and the cathode outlet and the second dryer can be connected to the other of the cathode inlet and the cathode outlet, wherein the connections of the dryers can be periodically switched between the cathode inlet and the cathode outlet, whereby one dryer recovers moisture from the outgoing oxidant gas stream and the other dryer humidifies the incoming oxidant gas stream.
Preferably, the fuel cell includes an inlet three-way valve having a common port connected to the cathode inlet and first and second branch ports, connected to the first and second dryers respectively, and an outlet three-way valve having a common port connected to the cathode outlet and having first and second branch ports connected to the first and second dryers respectively, each of the inlet and outlet three-way valves being switchable to connect one of the branch ports thereof to the common port thereof and to close off the other of the branch ports, and the inlet and outlet three-way valves are interconnected, whereby when the inlet three-way valve provides communication between the first branch port thereof, to the common port thereof, the outlet three-way valve provides communication between the second branch port thereof and the common port thereof and when the first three-way valve provides communication between the second branch port thereof and the common port thereof, the second three-way valve provides communication between the first branch port thereof and the common port thereof.
Advantageously, a pump is provided between the inlet three-way valve and the cathode inlet, for displacing the incoming oxidant gas stream into the fuel cell.
While the invention is applicable to a single fuel cell, it is anticipated that the invention will usually be applied to a plurality of fuel cells configured as a fuel cell stack. In such a case, a cathode inlet and outlet are connected to respective inlet and outlet manifolds connected to each of the fuel cells.
A water separator can be provided between the cathode outlet and the second three-way valve separating water droplets from the gas flow. This can provide a source of water which can be used for various purposes. For example, this water can be used for separate humidification of gas flows.
A separate, co-pending application, Ser. No. 09/592,643, pending filed simultaneously herewith under the title xe2x80x9cWater Recovery in the Anode Side of a Proton Exchange Membrane Fuel Cellxe2x80x9d is directed to water recovery on the anode side of a fuel cell. Nonetheless, the present invention envisages that water or moisture recovery could be effected on both the cathode side and the anode side. In this case, the fuel cell is preferably adapted for use with hydrogen as a fuel.
In such a case, the fuel cell would then include: a recirculation conduit including a pump connected between the anode inlet and the anode outlet; and a water separator provided in the recirculation conduit, between the anode outlet and the pump, for separating water from fuel gas exiting the anode; and a main fuel inlet comprising a first fuel inlet connected to the recirculation conduit, for supply of fuel.
A branch conduit can be connected to the recirculation conduit with the dryer and the branch conduit and a vent outlet for the branch conduit. A shut-off valve can be provided in a branch conduit, for controlling flow and enabling purge cycles to be effected. Alternatively, the dryer can be provided in the recirculation conduit downstream from the water separator, so as to control the moisture level in the recirculation conduit.
A second fuel inlet can be connected to the branch conduit between the dryer and the vent outlet and a second shut-off valve can be provided in the branch conduit between the second fuel inlet and the vent outlet. This enables fuel to be supplied through the second fuel inlet, for effecting reverse flow of fuel through the dryer to recharge the dryer and to recover moisture therefrom by humidifying the fuel stream.
Another aspect of the present invention provides a method of recovering moisture from an outgoing oxidant stream and humidifying an incoming oxidant stream in a fuel cell, which comprises an anode to which fuel is supplied and a cathode through which an oxidant stream passes and an electrolyte between the anode and the cathode, the method comprising the steps of:
(i) passing an outgoing oxidant stream from the cathode through a first dryer to recover moisture therefrom;
(ii) passing an incoming oxidant stream through a second dryer to humidify the incoming stream with moisture previously trapped in the second dryer; and
(iii) periodically switching the incoming and outgoing streams between the first and second dryers whereby each of the dryers alternately effect each of steps (i) and (ii).